1. Field of the Invention
The present invention relates to styrene-butadiene latices suitable for use as binders in coated papers to be printed by the web offset process.
2. Description of the Prior Art
The term "paper" is traditionally applied to felted or matted sheets of cellulose fibers, formed on a fine wire screen, such as in the Fourdrinier machine, from a dilute water suspension. The fibers are bonded together as the water is removed and the sheet is dried. The tendency of cellulose fibers to bond together, when dried from a water suspension is basic to papermaking technology.
Most of the water used in the manufacture of paper is removed mechanically when the web is carried through a series of press rolls. The sheet progresses to the drier section at about 33% consistency. Most of the remaining water is removed by evaporation. However, small amounts of water, on the order of about 2.5 to 5.0% by weight remain in the paper. These amounts of moisture are difficult to remove, and provide the web offset paper with the flexibility that is essential for handling.
An extremely dry web of paper would be brittle and tend to break when subjected to stresses in the web offset printing press. Even if an abnormally dry web of paper were to pass through a press without breaking, it would increase in size from the absorption of moisture from the air and it would be difficult to maintain register.
Paper that is to be printed upon is ordinarily coated with a pigment and latex binder to provide a smooth surface upon which a glossy, brilliant, detailed inked image is applied. The latex binder must be able to impart high ink holdout so that an image of maximum vividness and intensity can be obtained with as thin a film of ink as possible. The requirement that a latex impart high ink holdout, coupled with the use of extra adhesive to obtain sufficient surface strength for web offset printing has often resulted in a non-porous paper surface.
The printing of paper on a high speed web offset press requires the ink to dry very rapidly. The primary means for drying and setting the ink film is through evaporation of the solvent from the ink. This is often accomplished in a press dryer by impinging a gas flame or a high velocity stream of hot air on the printed web. The temperature of the air in a heatset dryer generally varies from about 500.degree. to 600 .degree. F. The surface temperature of the web when it exits the dryer can reach temperatures as high as 375.degree. F.
When the printed web is suddenly exposed to high temperature, the moisture in the paper is explosively vaporized. The vapor pressure must be allowed to exit through the coating or it will generate internal stresses within the paper. If the internal bonds are too weak to withstand the sudden outrush of vapor, an eruption in the form of a blister occurs on the paper surface.
Conventional styrene-butadiene-bound coatings have high ink holdout, low porosity and a marked tendency to blister. Traditionally, polyvinyl acetate latex binders have been used to alleviate the blister problem in web offset papers. See Walsh et al, "Polyvinyl Acetate Latex," TAPPI MONOGRAPH SERIES No. 37, p. 98 (1975) and Stoutjesdijk et al, PAPER TECHNOLOGY, vol. 15, no. 4, pages 209 to 212 (1974).
Unfortunately, polyvinyl acetate latex has often been implicated as the cause of deposits on the papermaking machinery referred to as "white pitch" which occurs when a significant amount of waste paper and trimmings, known as "broke," containing the polyvinyl acetate latex is recycled in the papermaking process.
Efforts have been made to develop a blister resistant styrene/butadiene binder. U.S. Pat. No. 4,429,074 to Mishiba et al discloses a coating composition for paper which comprises a mineral pigment and a polymer latex which is the product of emulsion polymerization of an aliphatic conjugated diene monomer, an ethylenically unsaturated carboxylic acid monomer, and a monoolefinic monomer in a weight proportion of 24.5 to 50:0.5 to 5:45 to 75 in the presence of carbon tetrachloride and an alkyl mercaptan.
The aforesaid Mishiba et al patent discloses that the combination of carbon tetrachloride and alkyl mercaptan in the polymerization of a polymer latex decreases the smell on irradiation by ultraviolet rays, improves adhesive strength, water resistance and blister resistance of the paper coated with such composition. Mishiba et al also discloses that the use of alkyl mercaptan alone, enhances the formation of coagula in the produced polymer latices, deteriorates mechanical stability and generates an unfavorable smell on irradiation with ultraviolet rays.
Hagymassy et al, "An Investigation of the Web Offset Blister Problem", TAPPI, Vol 61 pages 59-62 (January 1978) discloses styrene-butadiene binders with improved room temperature porosity but the resultant coatings blistered at lower temperatures than coatings with polyvinyl acetate, and the polymer failed to melt at the same temperature as does polyvinyl acetate.
The use of organosulfur compounds, such as mercaptans, as molecular weight modifiers and plasticizers in emulsion polymerization systems, including those for producing styrene-butadiene copolymers, is disclosed in U.S. Pat. Nos. 4,064,337 and 4,245,072 to Uraneck et al, U.S. Pat. No. 2,316,949 to Garvey, and U.S. Pat. No. 2,543,845 to Fryling.